1. Field of the Invention
This invention relates to an apparatus for controlling the speed of an elevator by means of a non-circulating, reversible, static Leonard device.
2. Description of the Prior Art
A conventional apparatus is shown in FIG. 1, and includes dual thyristor converters 2A and 2B connected in parallel reverse and coupled to the output of a 3-phase A.C. power source 1. The armature 3 of a reversible D.C. motor M is connected to the output of the converters and drives a traction sheave 4. A cable 5 is reaved on the sheave and has an elevator car 6 attached to one end and a balancing weight 7 attached to the other end.
The firing angles of the thyristor converters 2A and 2B are controlled in accordance with a current command signal Io shown in FIG. 2, as is well known, whereby the voltage applied to the armature 3 is appropriately varied and the rotational speed of the D.C. motor is correspondingly controlled.
In this prior art apparatus, switching is effected from converter 2A to converter 2B in order to change the direction of the armature current I from Ia to Ib. When this happens a transient period Td is necessary for the switching operation to be reliably achieved without any interference between the converters. The armature current I does not flow during this period Td, which is thus called dead time. After the switching period Td the current Ib begins to flow, and as shown in FIG. 2 it has a very steep initial rise, as is well known in the art.
This steep armature current rise causes an undesirable mechanical shock which is transmitted to the elevator passengers. The severity of the shock depends on the magnitude of the armature current change .DELTA.I as it rises, and on the rate of increase dI/dt of the armature current Ib.